They also found that horses cause disturbance by trampling the ground, further helping weeds to thrive.

"There's some weeds of national significance. They're the top 20 weeds in Australia that we're worried about," Associate Professor Pickering said.

"And we found some of those germinated in the dung. So it's something we just need to be careful about and take into consideration.

There's some weeds of national significance. They're the top 20 weeds in Australia that we're worried about. And we found some of those germinated in the dung.

Associate professor Catherine Pickering

"And the other problem is of course that dung - and of course urine - contains a lot of nutrients.

"Our soils tend to be really low in nitrogen and phosphorous and they're both things that you get actually quite a lot of - surprisingly large amounts - in urine from horses.

"So trampling, weeds, nutrient addition, all of those are things that we need to consider when we're making decisions about access."

Associate Professor Pickering says the manure of other animals did not poses as big a threat as horse manure.

"It can be [problematic] but the issue is... we're bringing horses in parks, so they may have been fed in pastures that might have had weed feed in them and then we bring them into the park, float them to the park walk, then ride them in the park, and so that spreads them," she said.

"We don't have many other animals that are fed on pasture land or grain stores, then walking through the park."

Spread of weeds assisted by other species

Mark Coleman, the former president of the Mountain Cattlemen's Association of Victoria, says horses are not solely to blame, as many other native and introduced species also spread weeds.

He says riding horses in national parks can actually help control weeds.

"With the introduction of blackberry into Australia, which is a horrific weed, you couldn't get a better spread of blackberry than the emu, followed probably by the deer," he said.

Mr Coleman says managing weed spread in national parks was not simply a matter of controlling horses.

[Cattlemen] were... the eyes and ears of these areas and once we were removed you remove man out of management.

Former president of the Mountain Cattlemen's Association of Victoria Mark Coleman

"We were still the eyes and ears of these areas and once we were removed you remove man out of management," he said.

"I really feel sorry for some of these park rangers trying to manage what they have to with such small resources.

"It really is disheartening for us to see how badly they are deteriorating."

The Griffith University findings were published in the journal Ecological Management and Restoration.